This invention relates to an audio control circuit used in two channel stereo and four channel stereo systems. Specifically, a type of "bass" and "treble" controls responsive to a d.c. voltage are described.
Circuit means for changing the bass or treble in a high fidelity audio system are described in U.S. Pat. No. 3,863,293 and in other references. The earlier audio control systems required mechanical motion to operate variable potentiometers. The change in resistance is used in one potentiometer to effect a change in bass and in another potentiometer to change the treble.
Where it is desirable to change the bass or treble in a two channel stereo system, a set of controls must be supplied to each channel. It is therefore desirable to have an audio control system for changing the bass or treble that is responsive to a single mechanical motion. This can be achieved by mechanically coupling the bass and treble controls of a first channel to the bass and treble controls of a second channel.
In four channel quadrophonic systems mechanical coupling between controls is more difficult to implement and "tracking" between channels becomes difficult to maintain. Consequently it is desirable to have a bass and treble control that responds to a d.c. voltage. A common d.c. voltage can then be used to control the bass and treble of several distinct channels.
A system that operates by means of a d.c. control voltage must exhibit high noise immunity to prevent noise from being introduced into the signal channel. Where low level signals are being processed noise susceptibility is a particular disadvantage of d.c. control systems. Prior art systems also suffer from a limitation in the dynamic range of signal levels that may be controlled.
Audio control systems must also produce a minimum of signal distortion in order to preserve the signal fidelity. Consequently it is undesirable to include non-linear circuit elements or frequency sensitive circuit elements in an audio control system.
Bass and treble controls must also be independent of each other. In some prior art bass and treble control systems, a change in bass would produce a small change in the treble.